This invention relates to the preparation of microporous polysulfone membranes. More specifically, the membranes prepared according to this invention are isotropic, or homogeneous, in that the porosity of the membrane is uniform over the cross section of the membrane. In addition, there is no skin present on either the inner or outer surface of the membranes of this invention. These characteristics are achieved by using a novel combination of casting solution, precipitation solution and spinning conditions. Liquid-liquid membrane precipitation or coagulation is employed. Both hollow fiber and flat sheet membranes can be prepared according to this invention.
Polysulfone membranes and hollow fiber membranes are known to the art and have been prepared to a variety of specifications. However, prior art hollow fibers typically are asymmetrical, or anisotropic. This type of fiber comprises a "skin" at the inner and/or outer surface and a microporous understructure. For example, U.S. Pat. No. 3,691,068 (Cross) discloses an anisotropic microporous polysulfone polymer membrane having a barrier layer at a surface thereof and a more porous support layer integral with the barrier layer. Similarly, U.S. Pat. No. 4,029,582 (Ishii et al.) discloses poly(arylether-sulfone) semipermeable membranes having a thin dense layer and a porous supporting layer.
Skinless polysulfone hollow fibers are known to the art. U.S. Pat. No. 4,612,119 (Eguchi) discloses preparation of a polysulfone hollow fiber filter medium having substantially no skin layer in the outer and inner surface areas. Eguchi uses a dry-wet spinning process, with carefully controlled temperatures. Cabasso et al., "Polysulfone Hollow Fibers II. Morphology," J. Applied Polymer Science, Vol. 21, pp. 165-80 (1977), which also utilizes a dry-wet spinning method, reports that control of the extrusion/coagulation procedure allows the formulation of skinned, porous skinned and non-skinned fibers. In a dry-wet spinning process, the membrane casting solution is extruded into an air space and then is conducted into a liquid precipitation medium.